1. Field of the Invention
The present invention relates to a time correction control apparatus and a method of time correction control.
2. Description of the Related Art
A GPS (Global Positioning System) satellite is an Earth-orbiting satellite which orbits around six orbits of inclination angle of about 55° at an altitude of 20,000 km. The number of GPS satellites has been increased due to enlargement of receiving range of a GPS receiver and reduction of regional difference in receiving state. At present, the number of GPS satellites is 28.
A GPS signal is sent from the GPS satellite. Here, the GPS signal is a radio signal sent from the GPS satellite. The frequency of the GPS signal sent from the GPS satellite is basically 1575.42 MHz (name: L1 wave). A signal called commercial C/A code (Coarse/acquisition code) is added to the frequency. Here, the C/A code is a code which is encoded by PRN code (Pseudo Random Noise Code) which is a pseudo noise code. Different C/A codes are allocated to the plurality of (28, at present) GPS satellites. Therefore, if signals are reversely dispersed to respective satellites using inherent C/A codes, the satellites can independently receive the respective signals.
In addition, the C/A code is repeatedly sent in a cycle of 1 ms (i.e., 1.023 MHz) a signal of 1023 bits. Data in which twenty cycles of C/A codes are made as one bit is navigation data. Here, the navigation data is data including orbit information of the GPS satellite, time information and the like.
The time information included in the navigation data includes HOW (Handover word) data which is hour/minute/second information and WN (Week Number) data which is year/month/day information. The HOW data is included in each subframe of the navigation data, and is sent from each GPS satellite every six seconds. The WN data is included only in a subframe 1 of the navigation data, and is sent from each GPS satellite every 30 seconds.
There is a known technique in which internal time of a time correction control apparatus is corrected utilizing the time information included in this navigation data. For example, signals (GPS signals) sent from a plurality of measuring satellites (GPS satellites) are received, and a receiving position and a receiving time are precisely obtained (e.g., Japanese Patent Application Publication Laid-open No. H9-178870).
According to this technique, however, it takes six seconds or more to receive one subframe from the plurality of subframes constituting the navigation data. Thus, in order to receive all of the plurality of subframes, the receiving time is correspondingly increased and thus, the power consumption of the time correction control apparatus is also increased.
The internal time is reset or stopped in some cases due to battery exhaustion, erroneous setting of user and a program error of the time correction control apparatus. In this case, a constant more error (time difference) with respect to the time information included in the GPS signal is generated. There is also a fear that this time difference is increased to a unit of year/month/day. Therefore, it is necessary to precisely correct the internal time and to reduce the power consumption by correcting not only the hour/minute/second but also the year/month/day.